PLT-1759 - Auto-complete for !channels when posting messages. (#3890)

* Auto-complete for !channels when posting messages.

This is part 1 of the fix for PLT-1759 to make channels linkable.

Still to do:
- Make the !channels clickable when they appear in messages. This is
  blocked until PR #3865 is resolved as it looks like that refactors
  some of the code that would be touched by making this change.

- Unit tests. Again, I think the above referenced PR should be merged
  before tackling this.

* Fix style problems.

* Highlighting of !channel-names in messages.

This only identifies the !channel-name (not the display name). The
implementation of the auto-complete on channel names now needs to be
modified to convert to the channel handle before sending the message.

* Display !channel-name as !Display Name.

When we encounter !channel-name in a message, display it as a link using
the channel's actual name rather than it's handle (name).

* Match on names and display name, and use name.

* Autocomplete channels matching on both the name and the the display
  name.
* Use the name as the text we fill in instead of the display name. It's
  potentially a bit ugly, but it minimises complexity for now as
  otherwise we'd have to do complicated things to the message box.

* Fix style issues.

* Load more channels everywhere.

Whenever we load the list of channels, we should also load the list of
more channels. This is to enable auto-completing and auto-linking of all
channels whether or not the user is in them currently.

* Include more channels in the map for linking.

* Listen for channel list updates for autolinking.

* Remove accidental console.log.

* Autocomplete on more channels too.

* i18n for channel autocomplete.

* Link directly to channels in !channel mentions.

This currently does not work if you aren't a member of that channel.
Need to decide what the correct behaviour is in that case.

* Fix style issues.

* Show channel name and handle in suggestion.

* Match channels only at start or after space.

* Better matching in text-formatting.

Only match channels after a space-type character or at the start in the
posts list too.

* Move the route construction to make tests work.

Moves route-construction out of text_formatting.jsx and into utils.jsx
so that the unit tests work once again.

webapp/components/textbox.jsx

@@ -3,6 +3,7 @@
 
 import $ from 'jquery';
 import AtMentionProvider from './suggestion/at_mention_provider.jsx';
+import ChannelMentionProvider from './suggestion/channel_mention_provider.jsx';
 import CommandProvider from './suggestion/command_provider.jsx';
 import EmoticonProvider from './suggestion/emoticon_provider.jsx';
 import SuggestionList from './suggestion/suggestion_list.jsx';
@@ -35,7 +36,7 @@ export default class Textbox extends React.Component {
             connection: ''
         };
 
-        this.suggestionProviders = [new AtMentionProvider(), new EmoticonProvider()];
+        this.suggestionProviders = [new AtMentionProvider(), new ChannelMentionProvider(), new EmoticonProvider()];
         if (props.supportsCommands) {
             this.suggestionProviders.push(new CommandProvider());
         }